Wireless communication networks are typically comprised of a mobile switching center, base stations and mobile/portable-units ("mobile units"). The base stations are typically hardwired to the mobile switching center through communication lines such as optical communication lines. Each base station provides coverage for mobile units within a specified area called a "cell".
When a mobile unit is within a particular cell and requests access, the base station for that cell assigns a frequency channel pair to the mobile unit. The frequency channel pair is comprised of an "uplink" frequency channel which is used for transmitting signals from the mobile unit to the base station, and a "downlink" frequency channel, which is used for receiving signals at the mobile unit from the base station. The frequency channel pair assigned to a particular mobile unit can be thought of as a single frequency channel and is often described as such by those skilled in the art. A mobile unit, such as a cellular telephone, can communicate with other mobile units or hardwired units, through its communications with a base station.
The use of mobile unit wireless communication devices such as cellular telephones, Personal Communications Networks (PCN), wireless Private Branch Exchanges (PBX's), and wireless Local Area Networks (LANs) is rapidly increasing. However, the radio frequency spectrum which is used in wireless communications is essentially comprised of a finite number of frequency channels. Various channel allocation techniques have been employed for the efficient use of this limited resource.
Dynamic Channel Allocation (DCA) is a broad title for several techniques for efficiently using the radio frequency spectrum. In DCA techniques, channel pairs are not preassigned to base stations.
Present DCA techniques can be grouped into two categories: traffic adaption and interference adaptation. "Timid", "Aggressive", and "m-Persistent Polite Aggressive" ("m-PPA") systems are examples of interference adaptation techniques. These techniques are also called distributed techniques because they allow mobile units to make decisions concerning channel allocation.
In a "Timid" technique a mobile unit measures the interference signal level on a channel pair and seizes that channel pair if the level indicates that no mobile units within a certain area, called a "neighborhood", are using that channel pair. In an "Aggressive" technique, a mobile unit, after an unsuccessful attempt to find an unused channel pair, sends out a "seize" signal on a channel whose interference level indicates that only one other mobile unit in the seizing mobile unit's neighborhood is using the corresponding channel pair. The other mobile unit, called the disturbed mobile unit, receives the "seize" signal and then looks for another channel pair. The "m-PPA" technique, is similar to the "Aggressive" technique, however, if the disturbed mobile unit cannot find another available channel pair the seizing mobile unit releases the seized channel pair and searches for another channel pair.
For "Timid", "Aggressive", or "m-PPA" techniques to work well, the mobile unit needs to be able to measure all channel pairs to determine which pair to make an attempt on. This may not be feasible. Furthermore the "Aggressive" technique can become unstable because when one mobile unit disturbs another it may start a series of changes in channel pair occupancy.
The "m-PPA" technique appears to give the best results however this technique also has drawbacks. The delay introduced to see whether a disturbed mobile unit can find another channel pair may not be satisfactory. In addition, multiple attempts to seize channel pairs by multiple mobile units throughout a wireless network may create unsatisfactory levels of interference on many channels.
In traffic adaptation DCA techniques, channel pairs are assigned to or occupied by mobile units based on actual data concerning channel pair usage by mobile units. Traffic adaptation does not require the measuring of interference signal level but rather the reporting of data concerning actual channel pair usage. Maximum Packing DCA (MP DCA) is a theoretical optimum for a traffic adaptation technique. In MP DCA centralized information determines which frequency channel pairs are used by all mobile units in a network. Each time a mobile unit requests a frequency channel pair for communication, the best possible allocation scheme is determined and all channel pair allocations are updated. MP DCA requires centralized coordination and global information which is almost impossible to achieve in a network with a large number of channels and cells.